1. Field of the Invention
The present invention relates generally to optical disk drive focusing systems. In particular, the present invention is a method for focus capture on a magneto-optic disk drive.
2. Description of the Prior Art
Magneto-optic data recording technology combines the erasability features of magnetic data storage systems with the high data storage capacity of optical systems. A 5.25 inch magneto-optic disk can hold up to 600M bytes of information, 1000 or more times the amount of information that a similarly sized magnetic floppy diskette can store. Magneto-optic disks are also transportable and can be transferred between drives. Since the reading, writing and erasing operations are performed with light beams rather than magnetic heads, they have long life, high reliability, and are relatively immune to physical wear.
The principles of magneto-optic technology are well known. Information is digitally stored at bit positions on a magneto-optic disk. The orientation of the magnetic field at each bit position can be switched between a first or digital one state in which its north pole is oriented upward, and a second or digital zero state in which the magnetic field is reversed and the north pole is oriented downward. The orientation of the magnetic field at each bit position is selected by subjecting the bit position to a magnetic field of the appropriate polarity, and heating the bit position of the disk. The magnetic orientation of the bit position is "frozen" when the disk cools and returns to room temperature.
The magnetic fields of all bit positions in an unwritten disk will generally be oriented with north poles down to represent digital zeros. When writing information, the bit positions will be subjected to a write magnetic bias field and heated by a high intensity laser beam. The orientation of the magnetic fields at the written bit positions will reverse to north poles up. Bit positions are erased by subjecting them to an erase bias field of the opposite polarity, and again heating the bit position. The magnetic field orientation at the erased bit positions will then reverse and switch to north poles down.
Data is read from the optical disk using a low-power or read intensity laser beam. Because of the magneto-optic phenomenon known as the Kerr Effect, the polarization of a laser beam impinged upon the bit positions will be rotated as a function of the magnetic orientation of the bits. The polarization of laser beam portions reflected from bit positions on the optical disk is detected by opto-electronic detector circuitry. Signals from the detector circuitry are then processed to determine whether the bit position is representative of a digital one or zero.
Bit positions are aligned adjacent one another in an elongated servo track on the optical disk. The optical disk can include a single servo track which is spirally positioned on the disk, or a plurality of concentrically positioned servo tracks. The laser beam used to read, write and erase data at the bit positions is focused onto the disk by an objective lens. Optical disk drives of this type typically include a focus servo system for driving the objective lens about a focus axis to keep the laser beam in focus with the disk. A tracking servo system is used to drive the objective lens along a tracking axis perpendicular to the servo tracks, and to maintain the laser beam centered over a desired servo track.
Optical disks typically have an optical layer and a protective layer. The optical layer has magneto-optic properties and is where the individual bit positions reside. The protective layer is clear and protects the optical layer from dust, corrosion, and abrasion. Ideally, the disk should be perfectly flat so the laser beam can remain in focus on the optical layer as the disk rotates. Unfortunately, manufacturing processes cannot produce perfectly flat disks.
A measure of the degree to which the disk is not flat is referred to as vertical runout. As the disk rotates, the disk surface moves up and down with respect to the objective lens because of vertical runout. A plot of the distance between the objective lens and the optical layer as a function of time generally resembles a sine wave. The slope at each point on the plot represents the velocity or the rate of change of distance between the objective lens and the optical layer. Vertical runout is measured in terms of this velocity. The maximum vertical runout is usually included in a disk specification.
After the laser beam has been modulated by the individual bit positions, it is reflected from the optical layer and impinged upon an optical detector. The optical detector is a group of individual elements arranged in a predetermined pattern and upon which the modulated laser beam is impinged.
Circuitry coupled to the optical detector produces focus error signals. The focus error signals are derived from the reflected laser beam, and represent the distance and direction that the objective lens is displaced from proper focus. The focus error signals are processed by a closed-loop focus servo system to generate focus drive signals. The focus drive signals are applied to an actuator or a motor and cause the objective lens to be driven to a position which minimizes the focus error.
The focus servo system monitors the focus error signals and continually adjusts the objective lens position so the laser beam remains in focus with the optical layer. In other words, the focus servo system drives the objective lens about its focus axis so it follows the topography of the disk surface as the disk rotates. The focus servo system maintains a nearly constant relative distance between the objective lens and the optical layer.
The closed-loop focus servo system maintains focus within a given boundary (i.e. maintains focus as long as the lens is close enough to the proper focus position that a focus error signal, which accurately describes the focus condition of the lens, can be derived from the detector). If the objective lens position is outside the given boundary, the focus servo system can no longer maintain lens focus during closed-loop servo system operation. Lens focus must then be recaptured in an open-loop control mode. Typically, there are three situations during which focus capture is required. First, when the disk drive is powered up and the disk begins to rotate, the objective lens is in an arbitrary start up position and is out of focus with the disk. Second, when a new disk is inserted into the drive the objective lens returns to the start up position and focus must be recaptured. Third, the focus servo system can lose focus occasionally during normal closed loop operation and will require focus capture. An example of the third situation is when the disk drive receives external forces of a magnitude greater than a specified limit causing a displacement of the objective lens.
Prior art focus capture techniques have included using a "read signal" threshold to close the focus servo loop. In this technique, the objective lens is driven about its focus axis in an open loop control mode until it approaches a position where it is in focus with prerecorded test data. When the appropriate test data is read, the objective lens position is assumed to be within controllable limits of the focus servo system. The control loop is then closed for normal operation. Unfortunately, this technique works only with sample disk media containing pre-recorded information. Composite "grooved" media may not contain any pre-recorded information and cannot be used in a magneto-optical disk drive using this focus capture technique.
A focus capture technique is also shown in Silvy et al. U.S. Pat. No. 4,700,056. This technique takes advantage of the imperfections in the manufacture of optical disk servo tracks. The radial position of a given servo track from its rotational axis is not constant all the way around the disk. The radial position (i.e. runout) can vary between 20 and 50 .mu.m per servo track revolution. Focus capture is performed with a focus servo system in an open-loop mode and an objective lens fixedly positioned along a tracking axis. The objective lens is cyclically driven along a focus axis about a neutral position and between offset positions. Displacement of the offset positions from the neutral position are increased with each successive cycle. At each offset position the number of sensed track crossings resulting from the eccentricity of the servo track are counted during a specified period. Focus capture is recognized when at least a predetermined number of track crossings are counted.
Another focus capture technique, called "fast scan", drives the objective lens along its focus axis and monitors the focus error signal for a condition indicating proper focus. Since the optical disk is formed with an optical layer and a protective layer, portions of the beam are reflected from each layer. Each of the reflections will generate a response in the focus error signal. Focus capture must focus on the optical layer and not on the protective layer. The "fast scan" approach starts the objective lens at a greatest distance from the disk and drives the lens toward the disk at a speed sufficiently greater than the expected vertical runout velocity that only one response will be produced from an in-focus condition on the protective layer. The second response received is then assumed to be produced by the optical layer, and the objective lens is stopped at this position. The closed loop focus servo system then takes control of the objective lens and maintains focus.
The fast scan technique is sufficient for lower performance optical disk drives but is not sufficient for high performance disk drives. Disk drive performance (data access speed) is improved by increasing the rotational velocity of the disk. Increasing the disk rotational velocity causes a proportional increase in the maximum vertical runout velocity. In higher performance magneto-optical disk drives, the relative velocity between the objective lens and the optical layer is too large for the fast scan technique. When the optical layer in-focus response is detected, the objective lens can be moving too fast relative to the optical layer surface to stop and enter the closed loop control mode before the optical layer moves out of the focus servo system control limits.
It is evident that there is a continuing need for improved focus capture techniques. A focus capture method which is effective with high performance magneto-optical disk drives would be especially desirable.